This invention relates to a device characterized in the definition of the species of claim 1 and a method for accomplishing the same.
Optical inspection of open beverage cans is performed, for example, in the bottling industry to sort out damaged, defective and/or soiled cans.
With the device of this type known from International Patent WO 96/31768, the collar on the inside of the can is illuminated obliquely from the outside above and imaged in a camera whose its optical axis is aligned with the axis of the can by using a reflector so that the path of the curve of the collar appears elongated. At the same time, the edge flange of the can is imaged separately. At another station of this device, the illuminated bottom of the can is imaged and examined together with an image of the inside lower wall area of the can. The light source is a truncated conical arrangement of LEDs above and outside the reflector.
Beam splitter mirrors and cameras arranged at the side are used for imaging the bottom and flange areas. Only a small portion of the incident light is reflected to the reflector. The inside surface of the collar facing outward and down cannot be imaged satisfactorily because a large portion of the light directed inward at the inside wall of the collar from outside and above at an inclination is reflected downward and is consumed by multiple reflection.
With the device known from International Patent WO 91/06846, the can having a bottom with a concave curvature is illuminated from the outside by means of a light source with a dome-shaped three-dimensional LED arrangement. A fisheye lens is arranged inside the light source. The various sections in the interior of the can are illuminated at different intensities, with the lighting intensity at the bottom of the can being lower than that on the inside wall of the collar of the can because of multiple reflection. Beams of light directed downward at an inclination are reflected repeatedly except for the bottom of the can. Beams of light directed vertically strike the bottom of the can directly. Direct light as well as reflected light striking the bottom of the can is reflected axially outward, with the light that is reflected outward and up at an angle from the inside bottom being brought to the lens by multiple reflections and over the inside wall of the collar. Therefor, a substantial portion of the incident light is lost. The image of the inside wall of the collar is not sufficiently informative. The light goes astray in the interior of the can, is absorbed to a great extent and produces such diffuse lighting of defects that they are hardly perceptible in the image.
With the device known from International Patent WO 98/19150 for the inspection of bottles, opaque bottles are illuminated from the outside to inspect the mouth sealing area and the outer thread area of the mouth surface. The reflected light is thrown by a pyramid-shaped or a round, dish-shaped optical conical reflector onto the lens of the camera. The conical reflector operates according to the optical principal of total reflection so that it advantageously does not need any opaque reflector surfaces that would be susceptible to damage.
The object of the present invention is to provide a device of the type defined initially and a method with which an informational image of the inside wall area of the collar that is critical with regard to damage and/or soiling is possible.
With this device, the light source is displaced virtually with this optical device to the bottom of the can through the opening in the can without any mentionable loss of light on the path to the bottom of the can for lighting the inside wall of the can. With the light source virtually displaced to the bottom of the can, the inside wall of the collar is illuminated intensely and uniformly from beneath thanks to the reflective properties of the bottom of the can in order to create an informational image from the reflected light coming from the inside wall of the collar. In this way, a considerable portion of the light can be utilized. Defects, soiling or residues on the collar can be detected easily and quickly; this is especially important for a bottling instillation operating at a high speed in order to sort out any damaged or soiled cans before they can cause a production interruption, or at least to be able to mark such a can.
According to the present invention, the procedure followed is that first no mentionable attention is devoted to the actual section of the can to be imaged, namely the inside wall of the collar, when it is illuminated but instead essentially the incident light directed into the can from the outside is bundled and passes through the narrow section of the reflector and then is directed through the opening in the can only at the bottom of the can. In order to be able to produce a uniformly illuminated and geometrically precisely defined light spot on the bottom of the can, by means of which the inside wall of the collar which is inclined radially inward can be illuminated effectively from beneath, the light which is at first bundled is then expanded accordingly. The reflected light coming from beneath and reflected on the inside wall of the collar reaches the camera, which then produces an informational image of the critical inside wall area of the collar thanks to an illumination quality similar to that which would be achieved if the light source were positioned on the bottom of the can.
Preferably a central circular or annular light spot is produced on the bottom of the can, representing the light source, and it should not be any larger than the outside diameter of the bottom of the can. In order to be able to utilize the largest possible cross section of the reflector with regard to a high reflection light yield, only a small opening in the reflector is used for illuminating the bottom of the can with bundled light, but this is not expanded optically after the narrow section to image a uniformly illuminated light spot on the bottom of the can which has a concave curvature and to utilize its reflective properties.
To achieve a sufficient light intensity even through a very small narrow section, beam shaping means should be provided between the light source and the narrow section for bundling and/or adjusting the beam outline to the outline of the bottom of the can.
If light of a traditional light source were bundled so that it could easily pass through a hole 5 to 10 mm in size, the light intensity that could be achieved would be too weak. However, if the light intensity of the traditional light source is strong enough, the light will diverge greatly and will strike the optical components of the reflector, thus producing reflection and strong overexposure and/or ghost images. Therefore, the light source is preferably a laser, in particular a semiconductor laser which delivers a bundled beam of light that passes through the narrow section(s) of the reflector without any loss. A collimator lens should be provided with the laser light source to bundle or focus the laser light beam. Then a cylindrical lens is expedient in order to change the astigmatic beam contour into a round contour which is used in the light spot on the bottom of the can. In focusing, the focal point is located within the range between the cylindrical lens and the interior of the reflector, depending on the setting. The narrow section may be a circular orifice with a diameter of 5 to 10 mm, for example.
Since modern laser light sources are very small, it would be conceivable to position a laser light source centrally above or in the narrow section.
However, the laser light source is preferably positioned at the side outside the path of the reflected light of the reflector directed toward the camera, and the bundled and focused laser light beam is reflected through a deflecting mirror through the narrow section.
It may be fundamentally advantageous to use or design the laser light source positioned centrally or the reflecting mirror as shading elements in order to avoid glare at the middle of the image that would interfere with the analysis of the image.
To widen the bundled and focused beam appropriately, a scattering lens should be provided downstream from the narrow section, preferably inside the reflector.
If a deflecting mirror is provided, it can easily be mounted on the reflector with a mount above an orifice of the reflector which defines the narrow section and optionally adjusted there.
The reflector, preferably a conical reflector, should preferably be covered by a protective glass to keep out soiling. The scattering lens can be mounted easily on the protected safety glass. The lens and the deflecting mirror, however, may also be mounted on a hollow tube (mirror on the upper end, lens on the lower end) which is mounted in the orifice of the reflector and has an inlet opening for the laser beam at the side next to the deflecting mirror.
According to this invention, the bundled and shaped laser light beam is expanded optically only downstream from the narrow section in such a way as to yield a uniformly illuminated light spot of a certain size and shape centrally on the bottom of the can. Since the widening takes place only downstream from the narrow section, reflections and severe overexposure and/or ghost images in the path of the beam to the narrow section can be avoided.